<Original Paper>Effect of the Application of Rice Straw and (NH_4)_2SO_4 to a Paddy Field on the Competitive Nodulation Ability of Indigenous Bradyrhizobium Strains
Competitive nodulation abilities of indigenous Bradyrhizobium strains in a paddy rice field at Osaska Prefectural Agricultural and Forestry Research Center were estimated by measuring both their nodule occupancy after inoculation of the soil suspensions to soybean and N-phenyl-1-naphthylamine (NPN) uptake activity of the isolates from the soils. Three plots (50 m^2 each) in the field had been applied annually for 18 years with (NH_4)_2SO_4 and rice straw at the following rate. Plot I : 0 and 0 kg, plot II : 100 and 0 kg, plot III : 100 and 7000 kg ha^<-1> y^<-1> , respectively. The inoculation test showed that bradyrhizobial population from the plot II had the highest competitive nodulation ability against B. japonicum strain 138 NR, and that from the plot I was the lowest. There was no significant difference in diversity of bradyrhizobial population among three plots in respect to intrinsic antibiotic resistance of the isolates of Bradyrhizobium from these plots. However, the plot II contained the highest proportion of the isolates which have large NPN uptake activity, indicating high competitive nodulation ability of the isolates. These findings were consistent with the previous results which indicated an increase in the competitive nodulation ability of a strain of B. japonicum under oligotrophic conditions
